If someone I know texts me [which hasn't happened yet b/c the cellphone I wish I didn't own doesn't have texting] or emails or calls me or I see him on the street and neglect to acknowledge him - to me [and this just "to me" - not הלכה למשה מסיני or anything like that] it is my way of saying "As far as I am concerned - you don't exist".
I don't think that is very kind and thus I hope not to be guilty of this in the future.
When I DO acknowledge the person then I am affirming his existence and value to me and the world. It is hard to think of very many things more important than that. There are 8 billion of us and the common denominator is that each of the 8 billion wants to be valued.
Where I live, it is not accepted for a man to say hello to a lady on the street or even acknowledge her, no matter what [unless she is having cardiac arrest or something and you are a Hatzala guy]. Once, I heard a woman calling out my name on the street on a dark night and I thought "Weird!" Then I realized I was married to her. But despite the fact that it is custom - I still consider it rude. I understand the reason behind it i.e. tzniyus and respect and follow it - קדושים תהיו is at the top of the pyramid and Rashi defines it as separating from Arayos - but still, a human being you know and you ignore them.... Hard pill to swallow.
Mussar Haskel: You decide for yourself...
